Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes directed to obtaining a lubricant base stock of suitable boiling point, viscosity, Viscosity Index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Aromatic components lead to high viscosity and extremely poor viscosity indices. Consequently, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been removed. Paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as phenol, furfural, N-methylpyrrolidone or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting (PDA) step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock.
Following the aromatics extraction step, dewaxing is normally used in order to improve the low temperature fluidity characteristics of the oil. Fluidity of a lubricant is normally determined by the pour point (ASTM D97). Generally, low pour point is desirable in order to ensure that the lubricant remains sufficiently fluid to function properly at the low temperatures which may be encountered in operation. Dewaxing may be carried out using conventional solvent dewaxing techniques, for example, solvent dewaxing with mixtures of methylethyl ketone (MEK) and toluene or by auto-refrigerant type solvent processes using liquid propane. Catalytic dewaxing is a highly satisfactory alternative to solvent dewaxing and a number of catalytic dewaxing processes are in commercial use. The Mobil lube oil dewaxing process (MLDW) is described in Catal. Rev. Sci. Eng. 28 (2 and 3) 185-264 (1986), especially 244-247. See also 1986 Refining Process Handbook, Gulf Publishing Co. (Sept., 1986, Hydrocarbon Processing) page 90.
Other treatment processes may also be employed to improve the properties of lubricants produced by refining techniques such as these, including hydrotreating to remove unsaturated compound color bodies and heteroatom-containing impurities, especially sulfur.
The desired viscosity for each lubricant product is determined by its intended use. The range of product viscosities will extend from light or less viscous oils such as the spindle oils and light neutral oils to heavy or viscous oils such as heavy neutral or bright stock. This range of viscosities is generally obtained by using fractions of different boiling point from the vacuum distillation tower. The lower boiling fractions are usually highly paraffinic and of low viscosity and these are typically used to produce the less viscous oils such as light neutral. More viscous, but still highly paraffinic oils are produced by using higher boiling distillates and the most viscous oils of all are produced from the more aromatic residual fractions which contain larger proportions of aromatic components resulting in the higher viscosity. Thus, in conventional refining techniques the viscosity of a lubricant is determined at least to some extent by its chemical composition. Certain viscous lubricants, however, may need to possess characteristics which are inconsistent with their normal chemical composition. For example, if a lubricant of marked stability is required, it is generally desirable to avoid the presence of aromatics which lead to poor oxidation and thermal stability. However, if this lubricant is also required to possess a relatively high viscosity, conventional refining techniques will introduce significant quantities of aromatic component. The formulation of certain types of lubricant has therefore been a compromise to satisfy such conflicting requirements.
The use of peroxide treatments for modifying the properties of various lube stocks including distillates and hydrocracked resids has been described in U.S. Pat. Nos. 3,128,246 and 3,594,320. Other peroxide treatment processes used with lubricants of synthetic origin are described in U.S. Pat. Nos. 4,594,172 and 4,618,737. As described in U.S. Pat. No. 3,128,246, treatment with peroxide improves the high temperature characteristics of the oil and in addition, raises its viscosity. Thus, this proposal provides some potential for controlling the viscosity of a lubricant independently of its chemical composition. However, the simple peroxide treatment is attended by a number of difficulties. The most significant of these is of controlling the treatment process in order to obtain products of high quality and predictable viscosity and Viscosity Index.
Viscosity Index (VI) is the most common measure that is applied to the decrease in viscosity of petroleum oils with increasing temperature. A series of Pa. oils exhibiting relatively small change in viscosity with changing temperature is arbitrarily assigned a VI of 100, whereas a series of Gulf Coast oils whose viscosities change relatively greatly is assigned a VI of 0. From the viscosity measurements at 40.degree. and 100.degree. C., the VI of any oil sample can be obtained from detailed tables published by the ASTM (ASTM D 2270). 14 Kirk-Othmer Encyclopedia of Chemical Technology, 489, (Wiley, 1981).
It has been found that the treatment of a lubricating oil stock by the addition of peroxide at the inlet of a continuous flow reactor results in both an expected increase in viscosity and an unexpected improvement in the Viscosity Index with no adverse effect on pour point.